Method and apparatus for disturbance detection

ABSTRACT

A sweep sensor may include a signal source, a propagation medium, and a detector. By transmitting an interrogating signal from the signal source into the propagation medium, detectable disturbances along the medium can physically alter the characteristics of the medium, which may cause a measureable change in the backscattered signal at the detector. Based on the change, it may be possible to locate the geographic origins of the physical disturbances along the propagation medium, or to determine the nature of the disturbances, or both. For example, it is generally possible to estimate the approximate distance between the detector and the disturbance given the time required to obtain the backscattered signal and the velocity of the signal source in the propagation medium. Further, in some embodiments, it is possible to quantify the amount of disturbance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/016,814 entitled “Method and Apparatus for Disturbance Detection”filed on Jan. 28, 2011, which claims the priority benefit of U.S.Provisional Application No. 61/337,176 entitled “Fiber optic sensor”filed on Jan. 30, 2010, and is related to the commonly assigned U.S.Pat. No. 7,725,026 filed on Apr. 1, 2005 entitled “Phase responsiveoptical fiber sensor” and U.S. Provisional Application Nos. 61/000,968entitled “Fiber optic intrusion detection system based on coherent OTDR”filed on Oct. 30, 2007, 61/065,600 entitled “Distributed fiber opticperimeter intrusion detection system with failover capability” filed onFeb. 13, 2008, 61/069,496 entitled “Distributed michelson intrusiondetection sensor” filed on Mar. 14, 2008, 61/195,762 entitled“Distributed fiber intrusion detection sensor” filed on Oct. 10, 2008,61/195,763 entitled “Local time series reconstruction for distributedfiber sensor” filed on Oct. 10, 2008, 61/207,274 entitled “Distributedfiber optic sensor” filed on Feb. 10, 2009, 61/283,019 entitled“Wavelength sweep fiber optic sensor” filed on Nov. 25, 2009, and61/335,575 entitled “Wavelength sweep fiber optic sensor” filed on Jan.8, 2010, each of which is hereby incorporated by reference in itsentirety.

BACKGROUND

Fiber optics may be used as a waveguide for a coherent light wavepropagating over a long distance. The refractive indices of the fiberoptics may be different along the cable, and cause the light wave totravel at non-uniform speeds. Inherent built-in defects, physicalperturbation, and temperature fluctuations may cause the refractiveindices of a fiber optic to change. As a coherent light wave propagatesthrough the fiber optics, the localized change of refractive indices mayalter the speed of the light wave, which results in phase changes, andmay cause the direction of propagation to reverse through scattering.Additionally, the birefringence of the fiber optic may cause the stateof polarization of the light wave to change. In a fiber optic system,the input of the fiber optics may be aligned, both in space and theangle of approach, with output of the light source to facilitate theefficient injection of the light wave. More than one fiber optics may befused together at their ends to increase the overall length of thewaveguide.

Fiber optics can provide propagation mediums for single mode ormulti-mode lasers, and are inherently immune to electromagneticinterference. For single mode transmission, the fiber optic line mayhave a smaller diameter than the fiber optic line for multi-modetransmission.

A fiber optic cable includes at least one fiber optic, typically madefrom the transparent glass fiber, in its core, and surrounded bytransparent cladding with a lower index of refraction. The cable furtherincludes a protect sleeve covering the cladding to minimize physicaldamages to the fiber optic line. A single cable can include one or morefiber optics. Fiber optics have the advantage of being a low-losswaveguide, and can relay optical signals over a long range without theneed to amplify the signals. Some fiber optics may be doped withdifferent materials for specialized purposes.

SUMMARY OF SELECTED EMBODIMENTS

A sweep sensor may include a signal source, a propagation medium, and adetector. By transmitting an interrogating signal from the signal sourceinto the propagation medium, detectable disturbances along the mediumcan physically alter the characteristics of the medium, which may causea measureable change in the backscattered signal at the detector. Basedon the change, it may be possible to locate the geographic origins ofthe physical disturbances along the propagation medium, or to determinethe nature of the disturbances, or both. For example, it is generallypossible to estimate the approximate distance between the detector andthe disturbance given the time required to obtain the backscatteredsignal and the velocity of the signal in the propagation medium.Further, in some embodiments, it is possible to quantify the amount ofdisturbance. In some implementations, the sweep sensor may be deployedas an intrusion detection sensor around the perimeter of a restrictedarea, for example an airport, a nuclear power plant, or a military base,to alert the security personnel of any unauthorized entry. Suchimplementations may place the propagation medium underground or along afence. In other implementations, the sweep sensor may be disposedsubstantially close to fluid-transporting pipes, such as petroleum ornatural gas pipes, to detect encroachment, excavation, leaks, andbreaks. In another implementation, the sweep sensor may be deployed as atemperature sensor for applications such distributed fire detection,pipeline leak detection, or downhole sensing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an implementation of a sweep sensor.

FIG. 2 shows another implementation of a sweep sensor.

FIGS. 3A-C show examples of the wavelength sweep cycle transmitted by asweep

FIGS. 4A-E show examples of transmitted interrogating pulses anddetected traces of a sweep sensor system.

FIGS. 5A-E show examples of temperature measurement and correlationmeasurement based on the detected traces.

FIGS. 6A-C show three example implementations of a light source used ina sweep sensor.

FIG. 7 illustrates an implementation of a sweep sensor which includespolarization management.

FIG. 8 shows example signals with polarization fading and withpolarization management to reduce fading.

FIG. 9 illustrates yet another implementation of a sweep sensor.

FIG. 10 illustrates another implementation of a sweep sensor.

FIG. 11A is a block diagram illustrating a method of Local Time SeriesReconstruction.

FIG. 11B shows an example diagram of collected pulses for the Local Timeseries Reconstruction.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A sweep sensor may include a signal source, a propagation medium, and adetector. By transmitting an interrogating signal from the signal sourceinto the propagation medium, detectable disturbances along the mediumcan physically alter the characteristics of the medium, which may causea measureable change in the backscattered signal at the detector. Basedon the change, it may be possible to locate the geographic origins ofthe physical disturbances along the propagation medium, or to determinethe nature of the disturbances, or both. For example, it is generallypossible to estimate the approximate distance between the detector andthe disturbance given the time required to obtain the backscatteredsignal and the velocity of the signal source in the propagation medium.Further, in some embodiments, it is possible to quantify the amount ofdisturbance. In some implementations, the sweep sensor may be deployedas an intrusion detection sensor around the perimeter of a restrictedarea, for example an airport, a nuclear power plant, or a military base,to alert the security personnel of any unauthorized entry. Suchimplementations may place the propagation medium underground or along afence. In other implementations, the sweep sensor may be disposedsubstantially close to fluid-transporting pipes, such as petroleum ornatural gas pipes, to detect encroachment, excavation, leaks, andbreaks. In another implementation, the sweep sensor may be deployed as atemperature sensor for applications such as distributed fire detection,pipeline leak detection, or downhole sensing.

In many traditional fiber optic sensors for detection, the sensitivityand/or length of the sensor may be limited by its signal-to-noise ratio(SNR). A sensor using a single mode light source may detectrandomly-generated noises in the backscattered signal originating fromthe optical source fluctuations and/or inherent defects in opticalfiber. Such noises can mask the intended signal and decrease the SNR,effectively limiting the sensitivity and range of the sensors. In someimplementations, the sweep sensor includes a wavelength-tunable lightsource that transmits interrogating signals of different wavelengthsinto the detection area. As such, the SNR of the sweep sensor may begreater than 100:1, and in particular embodiments, greater than 1000:1.

Referring to an example sweep sensor 100 shown in FIG. 1, a coherentlight source 110 transmits a wavelength-tunable interrogating signalinto a first optical fiber 120 a. The two ends of the first opticalfiber 120 a are coupled separately to the coherent light source 110 anda first terminal 132 a of a coupler 130. The interrogating signalentering the first terminal 132 a can exit the coupler 130 via a secondterminal 132 b and inject into a second optical fiber 120 b. A portionof the second optical fiber 120 b can be disposed in a detection area160. In some circumstances, the interrogating signal may propagate alongthe second optical fiber 120 b until contacting a first optical fiberterminator 140, which substantially reduces back reflections at the endof the second optical fiber 120 b. In some implementations, physicaldisturbances and/or defects in the second optical fiber 120 b can causesome of the interrogating signal in the second optical fiber 120 b tobackscatter in the opposite direction of the interrogating signals.Accordingly, the backscattered signal propagates along the secondoptical fiber 120 b back into the second terminal 132 b of the coupler130. Similar to the first interrogating signal, the backscattered signalcan exit a third terminal 132 c and into a third optical fiber 120 c,and propagates until reaching a detector 170. By evaluating thecorrelation among the backscattered signals, it is possible to determinethe presence, and the location of physical disturbances in the detectionarea 160.

Referring now to another example sweep sensor 200 shown in FIG. 2, insome implementations, a light source 210 includes a light emitter 212, amodulator 214, and an amplifier 216. The light emitter 212 can output acontinuous, wavelength-tunable coherent light wave into the modulator214. Examples of devices capable of performing the function of the lightemitter 212 include a semiconductor diode laser (e.g. III-V laser diode,vertical cavity surface emitting laser, or quantum well laser), aexternal cavity diode laser, a fiber laser, a solid state laser (e.g.Neodymium:Yttrium Aluminum Garnet laser), a gas laser (e.g. Helium Neonand Argon) or other appropriate lasers with suitable narrow spectrallinewidths. The light emitter 212 can output a continuous light waveinto the modulator 214, which may modulate the continuous light waveinto one or more similar or dissimilar pulses by periodically changingthe intensity of the continuous light wave. In some implementations, themodulator 214 may be an electro-optic device (e.g. Lithium Niobatedevice). Other possible devices for the modulator 214 may include anacousto-optic device (e.g. Bragg cell) or a semiconducting opticalamplifier used as an optical switch, for example. Next the pulsespropagate through the amplifier 216, which can increase the signalintensity of the pulses. Examples of the amplifier 216 can include adoped optical fiber amplifier or a semiconductor amplifier. Other typesof devices are possible. Subsequently, the light source 210 transmitsthe amplified pulses into a first optical fiber 220 a. As theinterrogating pulses propagate through the first optical fiber 220 a andinto a first terminal 232 a, a coupler 230 can transmit each individualinterrogating pulse to exit the coupler 230 via a second terminal 232 bto enter into a second optical fiber 220 b. In some implementations, thecoupler may be an optical circulator. In other implementations, thecoupler may be a 3-dB coupler. The interrogating signal inputting intothe second optical fiber 220 b can include one or more light pulses. Afirst optical fiber terminator 240 can minimize the reflected lightpulses from the end of the second optical fiber, thus reducing theintensity of light pulses reflected from outside a detection area 260.The first optical fiber terminator 240 may reduce the noise signalsinside the sweep sensor 200, for example. In some embodiments, the firstoptical fiber terminator 240 may include bending some portions of theend of the second optical fibers 220 b past its critical bend radius. Assuch, the interrogating signals reaching the first optical fiberterminator 240 can leak out of the second optical fiber 220 b instead ofreflecting back into the sweep sensor 200. In alternative embodiments,the optical fiber terminator 240 may be an optical isolator based on aFaraday rotator with polarizing elements. In some embodiments, thesecond optical fiber 220 b can be unterminated.

Still referring to FIG. 2, some embodiments of the sweep sensor 200 maydispose the second optical fiber 220 b in the detection area 260. Alength 262 of the detection area 260 may range from about 1 m to about100 km, from about 100 m to about 10 km, or from about 5 km to about 30km. Other ranges are possible. The pulse width and the transmissionfrequency of the interrogating pulses may depend on the length of thesecond optical fiber 220 b or on the length of the detection area 260.In some implementations, interrogating pulses propagating in the secondoptical fiber 220 b within the detection area 260 may backscatter fromlocations affected by detectable physical disturbances, which mayinclude temperature fluctuation or vibration near the second opticalfiber 220 b and distortion or displacement of the second optical fiber220 b. Backscattered light pulses can propagate along the second opticalfiber 220 b, in the opposite direction of the interrogating pulses, backto the coupler 230. Next, the backscattered light pulses can enter athird optical fiber 220 c through a third terminal 232 c, andsubsequently into a detector 270. In selective embodiments, the opticalfibers 220 may be single mode optical fibers. Examples of devices forthe detector 270 include one or more photodiodes, avalanche photodiodes,phototransistors, charge-coupled devices, pyroelectric detectors, andphotomultipliers. Other photo-detectors capable of sensing thebackscattered light pulses may be used.

In some implementations, the detector 270 may include a suitablephotodetector, an optical pre-amplifier, an electronic amplifier, asignal digitizer, and a processor. The processor may include acomputer-readable medium with a storage medium for storing data and aprocessing unit for analyzing the backscattered light pulses. In someimplementations, the detector 270 may include a plurality of detectors,arranged to implement diversity detection or differential or biaseddetection, for example.

In some implementations, during the operation of the sweep sensor, thelight emitter 212 may modulate one or more parameters of theinterrogating light wave. For example, as shown in a first graph 300 ofFIG. 3A, the wavelengths of the interrogating light wave may varydeterministically with time, from λ₁ at t₁, to λ₂ at t₂, to λ₃ at t₃, toλ₄ at t₄, and sequentially to λ₂₀₀ at t₂₀₀, and back to λ₁ at t₂₀₁ torepeat a wavelength sweep cycle. The wavelengths λ₁, λ₂, λ₃, λ₄ . . .and λ₂₀₀ may be evenly divided into a wavelength sweep range, i.e.λ₂=λ₁+Δλ, λ₃=λ₁+2Δλ, λ₄=λ₁+3Δλ, . . . λ₂₀₀=λ₁+199Δλ. In someimplementations, Δλ may range from about 1 femtometer to about 100femtometers, or about 5 femtometers to about 50 femtometers, forexample, or about 10 femtometers. Accordingly, the wavelength sweeprange may be from approximately 100 femtometers to approximately 10picometers, or approximately 600 femtometers to approximately 5picometers, and in a selective embodiment, about 1 picometer. Forexample, the wavelengths of the light pulse from the light source 210may transmit light waves centered at about 1500 nm. The wavelength rangeof the light can depend on the light emitter 212 or the modulator 214 orboth, and can include any suitable range which may provide a sampling ofconstructive and destructive interference conditions for locallybackscattered light waves.

As shown in the first graph 300 in FIG. 3A, such wavelength sweep cyclesmay begin with the shortest wavelength light, λ₁, and end with thelongest wavelength light λ₂₀₀, for example. Alternatively, asillustrated in a second graph 330 in FIG. 3B, the wavelength sweep cyclemay begin with the longest wavelength light, λ₂₀₀, and end with theshortest wavelength light λ₁, i.e. the wavelength of the light vary fromλ₂₀₀ at t₁, to λ₁₉₉ at t₂, to λ₁₉₈ at t₃, to λ₁₉₇ at t₄, andsequentially to λ₁ at t₂₀₀, and back to λ₂₀₀ at t₂₀₁. In anotherexample, the wavelength sweep cycle may alternate ascending anddescending sweepings of wavelengths (e.g. λ₁ at t₁ . . . λ₂₀₀ at t₂₀₀,λ₁₉₉ at t₂₀₁, and back to λ₁ at t₄₀₀). In a further example, the numberof wavelengths in the wavelength sweep cycle may range from about 100(e.g. λ₁, λ₂, λ₃, . . . λ₁₀₀) to about 160 (e.g. λ₁, λ₂, λ₃, . . .λ₁₆₀), from about 150 (λ₁, λ₂, λ₃, . . . λ₁₅₀) to about 240 (λ₁, λ₂, λ₃,. . . λ₂₄₀). Other variants are possible. In yet another implementation,the wavelengths in a wavelength range may be divided unevenly.

In some implementations, the wavelengths of the emitted interrogatinglight wave may be randomized by causing the instantaneous wavelength tochange in a nondeterministic or an asynchronous manner. An example ofsuch wavelength randomization is shown in a third graph 360 in FIG. 3C.

Now referring to FIGS. 2 and 4A, in some embodiments of the sweep sensor200, the light source 210 may launch one or more pulses with dissimilarwavelengths. An example of a portion of the interrogating signal, asillustrated in a first graph 400 in FIG. 4A, may include a number ofemitted pulses 405, or a packet 410, where each emitted pulse 405 in thepacket 410 can carry a generally different wavelength (e.g. a firstemitted pulse 405 aa has a wavelength of λ₁, a second emitted pulse 405ab has a wavelength of λ₂, a third emitted pulse 405 ac has a wavelengthof λ₃, a fourth emitted pulse 405 ad has a wavelength of λ₄ . . . and atwo hundredth emitted pulse 405 gr has a wavelength of λ₂₀₀). In anotherexample, the packet may include four hundred emitted pulses, where everytwo emitted pulses have substantially similar wavelengths. In yetanother example, one or more emitted pulses in a packet may each carrymore than one wavelength. Other combinations of emitted pulses andwavelengths are possible. During the operation of the sweep sensor 200,the light source 210 may send one or more packets 410 into the firstoptical fiber 220 a. Emitted pulses may backscatter in the secondoptical fiber 220 b in the detection area 260, and traverse back to thedetector 270 according to the path described previously, where thedetector 270 collects the backscattered pulses, or traces. The pulsewidths and duty cycles are exaggerated for illustrative purposes and mayvary according to the implementations discussed.

Still referring to FIG. 4A, in some implementations, the emitted pulses405 transmitted by the light source 210 may include a 100 nanosecondspulse. In other implementations, the pulse width may range from about 10nanoseconds to about 1000 nanoseconds, and in particular embodiments,may range from about 50 nanoseconds to about 200 nanoseconds. Otherpulse widths are possible. In selective embodiments, the shapes of theemitted pulses 405 in the packet 410 may be generally similar, andinclude a rectangular profile. In alternative embodiments, each emittedpulse 405 in the packet 410 may include substantially similar ordissimilar shapes, pulse widths, or separations from neighboring emittedpulses 405. Further, the packet 410 may include about 100 pulses toabout 160 pulses, about 150 pulses to about 240 pulses, and in aparticular embodiment, 200 pulses.

In some embodiments of the sweep sensor 200, the separation time betweenthe emitted pulses 405 may be related with the length of the sweepsensor 200. After the first emitted pulse 405 aa is transmitted into thefirst optical fiber 220 a, the second emitted pulse 405 ab may betransmitted after all backscattered light from the first emitted pulse405 aa has reached the detector 270 or has exited the detection area.Accordingly, the separation time can be substantially similar to theround-trip time (i.e. time for the emitted light in pulses 405 totraverse the sweep sensor 200, and return as backscatter to the detector270). Alternatively, the separation time between the emitted pulses 405may be longer or shorter than the round-trip time of light in the sweepsensor 200.

Now referring to FIGS. 2 and 4A-E, FIGS. 4B-E show in illustrativediagrams examples of detected traces 425, 445, 465, 485 constructed fromthe backscattering of the emitted pulses 405 in the sweep sensor 200 fora location along the second optical fiber 220 b within the detectionarea 260. The ordinates of a second, third, fourth, and fifth graphs420, 440, 460, 480 represent the intensities of the traces 425, 445,465, and 485, respectively, while the abscissas represent thewavelengths of the emitted pulses. Specifically, using the second graph420 of FIG. 4B as an illustrative example, each trace 425 may representthe backscattered intensities of emitted pulses 405 within the packet410, plotted against the wavelengths of the emitted pulses 405, for alocation along the second optical fiber 220 b. For example, to obtainthe first trace 425 aa, the light source 210 can send one packet ofinterrogating pulses, which can cover a wavelength sweep cycle, into thesweep sensor 200. The intensity of the backscattered signal collected atthe detector 270 after a fixed time interval following the emission ofeach pulse 405 can be used to construct the first trace 425 aa againstthe swept wavelength for a location along the second optical fiber 220 bwith light round-trip time corresponding to said fixed time interval.The spatial extent of the location so interrogated is related to thewidth of the emitted pulses and the bandwidth of the detector 270. Eachtrace 425 may represent a packet of backscattered pulses transmittedwith deterministically different wavelengths (e.g. one hundred packetssent into the sensor can generate one hundred traces). In general, ifthe light source 210 transmits two hundred emitted pulses 405, each witha different wavelength, then the detector may correspondingly receivetwo hundred backscattered pulses, which can construct one trace, forexample. The second graph 420 in FIG. 4B illustrates an example oftraces 425 when the second optical fiber 220 b experiences minimumdetectable physical disturbance at the said location. The traces 425substantially have similar shapes, and overlap in the relative timedomain. As such, the traces 425 in FIG. 4B are highly correlated, whichcan indicate minimum detectable physical disturbance at the saidlocation in the detection area 260.

Referring to the third graph 440 in FIG. 4C, the traces 445 havesubstantially similar shapes, and include lateral shifts along thewavelength domain (e.g. as is evident in comparison of the first trace445 aa to the third trace 445 ac). Such traces can be intermediatelycorrelated, and indicate slow drifting or uniform physical disturbances,for example, such as ambient temperature change, strain fluctuation,slow displacement or change in load condition near a location in thesecond optical fiber 220 b, represented by the traces 445 of the thirdgraph 440. Based on the shifts among the traces 445, it may be possibleto quantitatively determine the amount of temperature change, forexample.

Referring more closely to the fourth graph 460 in FIG. 4D, the first,second, ninety-ninth and one hundredth traces 465 aa, 465 ab, 465 du,465 dv have generally similar shapes, corresponding to substantiallysimilar initial and final steady-state conditions. The remaining traces465 are minimally correlated and may represent the backscattered signalduring the presence of fast physical disturbances. Specifically, thefirst and second traces 465 aa, 465 ab may indicate minimum detectablephysical disturbances at a location in the detection area 260. Thethird, and fourth traces 465 ac, 465 ad show random fluctuations in theintensities of the backscattered signals, which may indicate thepresence of physical disturbances occurring on the time scale comparableto or faster than the time intervals between the traces 465. Theninety-ninth and one hundredth traces 465 du, 465 dv substantiallyrestores to the generally similar shape as the first and second traces465 aa, 465 ab, which indicates the removal of fast physicaldisturbances. Such evolution may indicate the presence of reversiblefast physical disturbances, such as fast vibration, in a location in thedetection area 260. The magnitude of said physical disturbances may beestimated by the degree of correlation between trace 460, ranging fromsubstantially full correlation of 1 to minimal correlation of 0.

Referring now to the fifth graph 480 in FIG. 4E, the first and secondtraces 485 aa, 485 ab have a generally similar first shape, and theninety-ninth and one hundredth traces 485 du, 485 dv have a generallysimilar second shape. The remaining traces 485 are minimally correlatedand may represent the backscattered signal during the presence of fastphysical disturbances. Such evolution may indicate the presence ofirreversible physical disturbances, such as displacement or change inload conditions, at 10 a location in the detection area 260.

As stated above, examples of fast physical disturbances include fastvibration (e.g. a potential intruder walking near the second opticalfiber 220 b) and displacement (e.g. a potential intruder steppingdirectly on or digging underneath a buried optical fiber, or physicallytouching an above-ground fiber-optic cable). Other contributing factorsmay also cause the detected traces 465 to be minimally correlated. InFIGS. 4A-E, the shapes, durations, and wavelength shifts of the pulses405 and the traces 425, 445, 465, 485 are for purposes of illustrationand may vary. A number of known methods may be used to computecorrelation such as Pearson Correlation and Brownian Correlation, forexample. Other approaches to quantifying the differences between thedetected traces may also be used.

Referring to FIGS. 2 and 5A, the first temperature graph 500 shows anexample measurement of temperature change at a location along the secondoptical fiber 220 b in the detection area 260 by quantifying the shiftin the detected traces, such as the traces 445 shown in FIG. 4C.

Referring now to FIGS. 5B-E, as an example illustration of thefunctionality of an implementation of a sweep sensor, such as the sweepsensor 200 described in relation to FIG. 2, a mechanical machinery usedfor digging trench is disposed near the second optical fiber 220 bwithout making physical contact to the second optical fiber 220 b. FIGS.5B-E include a first, second, third, and fourth correlation graphs 520,530, 540, 550 which illustrate, respectively, a first, second, third,and fourth correlation curves 522, 532, 542, 552. During the measurementof the second correlation curve 522, as illustrated in FIG. 5B, themechanical machinery remains in the off state, in which the sweep sensor200 detects no disturbance (i.e. highly correlated traces). The third,fourth, and fifth correlation curves 532, 542, 552, as illustrated inFIGS. 5C-E, show example detections during three states of operation ofthe mechanical machinery: activation of motor, engagement of chainsaw,and digging, respectively, showing progressively reduced correlation inaccordance with the magnitude of disturbance.

FIGS. 6A-C show exemplary embodiments of light sources 600, 640, 670that may generate the interrogating pulses appropriate for the coherentlight source 110 shown in FIG. 1 (or the light source 210 in FIG. 2). Insome implementations, the light source 600 may include a light emitter610, a modulator 620, and an amplifier 630. The wavelengths of thecontinuous coherent light wave can be controlled in the light emitter610 by changing the drive current, device temperature, or cavity lengthof the light emitter 610, for example. Alternatively, the wavelengths ofthe light wave may be controlled at the modulator 620 by changing theelectric field applied to an electro-optic modulator, or changing thevibration frequency applied to an acousto-optic modulator. In someimplementations, the interrogating signal may include pulses modulatedby changing the drive current of the light emitter 610, the shutterstate of the modulator 620 (e.g. Q-switch), or the external stimuli(e.g. electric field or vibration) applied to the modulator 620 (e.g.electro-optic or acousto-optic) as described above. As the interrogatingpulses exit the modulator 620, the amplifier 630 may increase theintensities of the pulses when pumped optically or electrically by anexternal source (not shown). Examples of the amplifier 630 include adoped optical fiber amplifier (e.g. erbium doped optical fiberamplifier) and a semiconductor optical amplifier.

In other implementations, the light source 640 may include a lightemitter 650 and a modulator 660 as shown in FIG. 6B. For example, thelight emitter 650 may include a solid state laser described above, andthe modulator 660 can provide means for changing the wavelengths of thecontinuous light wave and generating the interrogating pulses. Forexample, the modulator 660 may include an acousto-optic device describedabove for the wavelength modulation, and an electro-optic device togenerate the interrogating pulses.

In additional implementations, as shown in FIG. 6C, the light source 670may include a light emitter 680 that directly emits interrogatingpulses, such as pulses with varying wavelengths for example. Othercombinations of the light emitter, modulator, and amplifier may generateappropriate interrogating pulses for a sweep sensor such as the sweepsensors 100 and 200.

Referring to FIG. 7, an example of a sweep sensor 700 may include alight source 710 and a polarization controller such as ones illustratedin U.S. Provisional Application No. 61/000,968 entitled “Fiber opticintrusion detection system based on coherent OTDR” filed on Oct. 30,2007, hereby incorporated by reference in its entirety. In someimplementations, the light source 710 emits an interrogating signal intoa polarization controller 718 for polarization management. Theinterrogating signal may include pulses with substantially differentwavelengths, as described above. Next, the interrogating signalpropagates into a first optical fiber 720 a. The two ends of the firstoptical fiber 720 a are coupled separately to the polarizationcontroller 718 and a first terminal 732 a of a coupler 730. Theinterrogating signal entering the first terminal 732 a can exit thecoupler 730 via a second terminal 732 b and injects into a secondoptical fiber 720 b. A portion of the second optical fiber 720 b can bedisposed in a detection area 760. In some circumstances, physicaldisturbances or defects in the second optical fiber 720 b or both in thedetection area 760 can cause some of the interrogating signal in thesecond optical fiber 720 b to backscatter in the opposite direction ofthe interrogating signals, while the remaining interrogating signal maypropagate along the second optical fiber 720 b until contacting a firstoptical fiber terminator 740. Accordingly, the backscattered signalpropagates along the second optical fiber 720 b back into the secondterminal 732 b of the coupler 730. The backscattered signal can exit athird terminal 732 c and into a third optical fiber 720 c, andpropagates until reaching a detector 770. As shown in an example graph800 in FIG. 8, adjusting the states of polarization (SOP) of theinterrogating signal may reduce polarization-induced signal fading ofthe backscattered signal. For example, the polarization-induced signalfading occurs within approximately a first time frame 810 when the inputcoherent signal has a first SOP 820. If the input polarization statechanges to a second SOP 830, the polarization induced signal fadingwithin the first time frame 810 can be avoided. Methods of managing theSOP include polarization scrambling, polarization modulation, andpolarization dithering, for example. In some implementations, theinterrogating pulses may include light waves of generally differentpulse profiles, such as wavelength, chirp, phase, pulse width, pulseshape, and SOP, as described in U.S. Provisional Application No.61/283,019 entitled “Wavelength sweep fiber optic sensor” filed on Nov.25, 2009 and 61/335,575 entitled “Wavelength sweep fiber optic sensor”filed on Jan. 8, 2010, each of which is hereby incorporated by referencein its entirety. For example, in a packet of interrogating pulses, eachpulse may include light wave of a generally different SOP. Furthermore,each pulse may include more than one distinct pulse profile (e.g.wavelength and SOP). In some implementations, a pulse may include morethan one wavelength or a variable wavelength chirp. Wavelength, chirp,phase, pulse width, pulse shape, and SOP may be modulated by the lightemitter, modulator, polarization controller or any combination of thethree devices. Other pulse profiles of the light wave may be altered foreach interrogating pulse in a packet.

FIG. 9 illustrates an example of a sweep sensor 900 such as onesdescribed in U.S. Provisional Application Nos. 61/069,496 entitled“Distributed Michelson intrusion detection sensor” filed on March 14 and61/207,274 entitled “Distributed fiber optic sensor” filed on February10, each of which is hereby incorporated by reference in its entirety.In some implementations, the sweep sensor 900 may include a light source910 which emits an interrogating signal formed substantially of pulseswith generally different wavelengths, as described above. Next, theinterrogating signal propagates into a first optical fiber 920 a. Thetwo ends of the first optical fiber 920 a are coupled separately to thelight source 910 and a first terminal 932 a of a coupler 930. Theinterrogating signal entering the first terminal 932 a of the coupler930 can be split into three interrogating signals with similar ordissimilar amplitudes. After the split, two of the three interrogatingsignals can exit the coupler 930 via a second and a third terminal 932b, 932 c, and inject into a second and a third optical fiber 920 b, 920c, respectively. In selective embodiments, portions of at least one ofthe second and third optical fibers 920 b, 920 c can be disposed withinthe same or different cables in a detection area 960. In somecircumstances, physical disturbances or defects in one or both of theoptical fibers 920 b, 920 c in the detection area 960 can cause some ofthe interrogating signals in the second and/or third optical fibers 920b, 920 c to backscatter in the opposite direction of the interrogatingsignals, while the remaining interrogating signals may propagate alongthe second and the third optical fibers 920 b, 920 c until contacting afirst and a second optical fiber terminator 940, 950, respectively.Accordingly, the backscattered signals propagate along the second and/orthird optical fibers 920 b, 920 c back into the second and/or thirdterminals 932 b, 932 c of the coupler 930, respectively. Thebackscattered signals can combine, interfere, and split into threebackscattered signals with similar or dissimilar amplitudes andpotentially with a different phase bias for each of the three splitsignals. One of the three split signals exits the first terminal 932 aand injects into the first optical fiber 920 a, while the other twosplit signals exit a fourth and a fifth terminal 932 d, 932 e and into afourth and a fifth optical fiber 920 d, 920 e. Each of the other twosplit signals propagates until reaching a detector 970. Detector 970 maybe configured to measure the relative phase between the twobackscattered signals combined at the coupler 930, as described in theU.S. Pat. No. 7,725,026, filed on Apr. 1, 2005, entitled “Phaseresponsive optical fiber sensor”, hereby incorporated by reference inits entirety.

In some embodiments, the sweep sensor 900 may be able to detect one ormore simultaneous disturbances in the detection area. For example, asthe split interrogating signals propagate along the second and thirdoptical fibers 920 b, 920 c, physical disturbances occurring in a firstand a second event site 962 a, 962 b can cause detectable differences inthe phase information of the backscattered signals along the length ofthe second and third optical fibers 920 b, 920 c. Specifically,disturbances at the first and second event sites 962 a, 962 b haveminimum effect on the phase information of the interrogating signalsbackscattered in a first sector 964 a. Disturbances at the first eventsite 962 a can change the phase information of the interrogating signalsbackscattered in a second and a third sector 964 b, 964 c as theinterrogating signals propagate through the event sites 962. Similarly,disturbances at the second event site 962 b can cumulatively change thephase information of the interrogating signals backscattered in thethird sector 964 c. In some implementations, the backscattered signalsinject into the detector 970 via the fourth and fifth optical fibers 920d, 920 e. By analyzing the phase information of the backscatteredsignals, it is possible to detect multiple disturbances in the detectionarea 960 with the sweep sensor 900. In other implementations, apolarization controller (not shown in FIG. 9) may be inserted betweenthe light source 910 and the first optical fiber 920 a to manage the SOPof the interrogating signals prior to entering the detection area 960.

In some embodiments, a sweep sensor may include two opposing detectionoptical fibers to detect phase information of the backscattered signal,such as ones described in U.S. Provisional Application Nos. 61/065,600entitled “Distributed fiber optic perimeter intrusion detection systemwith failover capability” filed on Feb. 13, 2008 and 61/195,762 entitled“Distributed fiber intrusion detection sensor” filed on Oct. 10, 2008,each of which is hereby incorporated by reference in its entirety.Referring to FIG. 10, a sweep sensor 1000 may include a light source1010 which emits an interrogating signal that includes pulses withsubstantially different wavelengths, as described above. Next, theinterrogating signal propagates into a first optical fiber 1020 a, theninto a first terminal 1032 a of a first coupler 1030 a, and subsequentlycan be split into two interrogating signals with similar or dissimilaramplitudes. After the split, the two interrogating signals can exit thefirst coupler 1030 a via a second and a third terminal 1032 b, 1032 c,and inject into a second and a third optical fiber 1020 b, 1020 c,respectively. The two interrogating signals exit a fourth and a fifthterminal 1032 d, 1032 e and inject into a fourth and a fifth opticalfiber 1020 d, 1020 e, which may be encased in the same or differentcables. A portion of the fourth and the fifth optical fibers 1020 d,1020 e can be disposed in a detection area 1060. In some circumstances,physical disturbances or defects in one or both of the fourth and fifthoptical fibers 1020 d, 1020 e in the detection area 1060 can cause someof the interrogating signals to backscatter in the opposite direction ofthe interrogating signals, while the remaining interrogating signals maypropagate along the fourth and the fifth optical fibers 1020 d, 1020 euntil contacting a first and a second optical fiber terminator 1040,1050, respectively. Accordingly, the backscattered signals propagatealong the fourth and fifth optical fibers 1020 d, 1020 e back into thefourth and fifth terminals 1032 d, 1032 e of the second and a thirdcoupler 1030 b, 1030 c, respectively. Next, the backscattered signalsexit a sixth and a seventh terminals 1032 f, 1032 g, inject into a sixthand a seventh optical fiber 1020 f, 1020 g, and propagate until reachinga first and a second detector 1070 a, 1070 b, respectively.

The configuration of the sweep sensor 1000, for example, may allow thecontinuingly proper function of the sweep sensor 1000 after anaccidental or intentional disabling of the sweep sensor 1000. Forexample, a potential intruder may sever the one or more cable(s)encasing the interrogating fourth and fifth optical fibers 1020 d, 1020e of the sweep sensor 1000. In an illustrated example, severing thecable(s) can disable the sensing capabilities of a portion of the fourthoptical fiber 1020 d, from a sever point 1065 to the first optical fiberterminator 1040, and/or a portion of the fifth optical fiber 1020 e,from the sever point 1065 to the second optical fiber terminator 1050.However, the failover configuration, where two sensing optical fibers(i.e. fourth and fifth optical fiber 1020 d, 1020 e) are disposedoverlappingly in the detection area 1060 in the opposite direction, canallow the sweep sensor 1000 to continue to detect physical disturbancesin the detection area 1060. Specifically, the fourth and fifth opticalfiber 1020 d, 1020 e can separately sense physical disturbances up tothe sever point 1065, and maintain the sensing ability of substantiallythe entire detection area 1060. In another embodiment (not shown) of thefailover configuration, a single sensing optical fiber may be used as aback-up sensor to a main sensor. For example, to implement the failovercapability to the sweep sensor 200 shown in FIG. 2, the two ends of thesecond optical fiber 220 b may connect directly to the second terminal232 b and a fourth terminal 232 d (e.g., in a loop). The single sensingoptical fiber may be disposed in the same cable as the main sensor,which can be one of the sweep sensors described above. As the apotential intruder severs the fiber cable, thus disabling a portion ofthe main sensor, an optical switch (not shown) may reroute theinterrogating pulses to the severed single sensing optical fiber of theback-up sensor, for example. As such, the noninterfering signals may bepropagated across the severed single optical fiber in oppositedirections, effectively functioning similarly to the sweep sensor 1000described in FIG. 10. Other configurations are possible, and may beintegrated with previously described sensors structures.

In some implementations, a single direction sweep sensor, such as thesweep sensors 200, 700, 900 described previously, can detect a break inthe sensing optical fiber, e.g. the second optical fibers 220 b, 720 b,920 b. More specifically, if a potential intruder cuts the secondoptical fiber 220 b in the sweep sensor 200, some of the interrogatingsignals may reflect at the sever point (not shown), instead ofpropagating to the first optical fiber terminator 240. The sever point,which may be terminated differently than design, can generate a largerreflected signal than the optical fiber terminator 240. The increase inreflected signal may lead to an alarm (not shown) to alert the operatorof the sweep sensor 200 regarding the potential sabotaging effort of thesweep sensor 200. Additionally, a break in the sensing optical fiber(e.g. the second optical fibers 220 b, 720 b, 920 b) may reduce thebackscatter time of the interrogating signal, which may be used todetect the presence of an intruder attempting to disable the sweepsensors 200, 700, 900, for example.

FIG. 11A shows an implementation of the local time series (LTS)reconstruction system such as ones illustrated in U.S. ProvisionalApplication No. 61/195,763 entitled “Local time series reconstructionfor distributed fiber sensor” filed on Oct. 10, 2008, the entirety ofwhich is herein incorporated by reference. A LTS reconstruction system1100 can facilitate data processing of the traces detected by thedetector 270 of the sweep sensor 200 shown in FIG. 2, for example. Asshown in FIG. 11A, the LTS reconstruction system 1100 includes adetector 1105, such as the detector 270 in the sweep sensor 200, thattransmits collected traces from the backscattering of the interrogatingsignals to a local time series constructor 1110. The local seriesconstructor 1110 stores the collected traces on a memory buffer 1115until a predefined amount of traces have been collected. Each trace, forexample, represents the response of a packet of interrogating pulsesbackscattered from the detection area 260 during one wavelength sweepcycle. By superimposing two or more collected traces and normalizing tothe detection area, a trace graph 1150 shown in FIG. 11B, may beconstructed to plot the traces with respect to specific physicallocations in the detection area (e.g., detection area 260). In someimplementations, the trace graph 1150 plots an intensity of signalagainst a distance, from one edge of the detection area to another edge.The normalized traces may be sent to a signal processor 1120, such asone capable of performing signal processing techniques such as Fourieror Wavelet Transformations, to detect physical disturbances in thedetection area. For example, applying the LTS reconstruction method tothe sweep sensor 200 may improve the sensitivity of the sweep sensor 200by minimizing or ignoring the contribution of noise.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A system for monitoring a detection area, comprising: a coherentlight source transmitting a first plurality of interrogating pulses,wherein the first plurality of interrogating pulses includes a firstsubset of interrogating pulses and a second subset of interrogatingpulses, the first subset of interrogating pulses comprising asubstantially different pulse profile in relation to the second subsetof interrogating pulses, the first subset of interrogating pulses beingarranged in a deterministic fashion in relation to the second subset ofinterrogating pulses; a first optical fiber of a first predeterminedlength disposed in the detection area; a first coupler disposed betweenthe coherent light source and the first optical fiber and coupled to thefirst optical fiber at a first terminal, wherein the first coupler isconfigured to: provide the first plurality of interrogating pulses tothe first optical fiber through the first terminal, receive at least afirst plurality of backscattered pulses generated in the first opticalfiber as reflections of at least a portion the first plurality ofinterrogating pulses, and provide the first plurality of backscatteredpulses through a second terminal; a second optical fiber of a secondpredetermined length coupled to the second terminal of the firstcoupler, the second optical fiber receiving the first plurality ofbackscattered light pulses from the first coupler; and a first detectorcoupled to the second optical fiber, the first detector configured to:receive the first plurality of backscattered pulses, and analyze thefirst plurality of backscattered pulses to identify a first location ofa first fast physical disturbance in the detection area.
 2. The systemof claim 1, wherein the first detector analyzes the first plurality ofbackscattered pulses based on intensity as a function of wavelength todetermine a correlation among the first plurality of backscatteredpulses.
 3. A method for detecting a disturbance comprising: transmittinga first plurality of interrogating pulses from a light source, the firstplurality of interrogating pulses including a first subset ofinterrogating pulses and a second subset of interrogating pulses, thefirst subset of interrogating pulses comprising a substantiallydifferent pulse profile in relation to the second subset ofinterrogating pulses, the first subset of interrogating pulses beingarranged in a deterministic fashion in relation to the second subset ofinterrogating pulses; transmitting at least a second plurality ofinterrogating pulses from the light source, the second plurality ofinterrogating pulses including a third subset of interrogating pulsesand a fourth subset of interrogating pulses, the third subset ofinterrogating pulses comprising a substantially different pulse profilein relation to the fourth subset of interrogating pulses, the thirdsubset of interrogating pulses being arranged in a deterministic fashionin relation to the fourth subset of interrogating pulses; transmittingthe first plurality of interrogating pulses and the second plurality ofinterrogating pulses into a first optical fiber disposed in a detectionarea, wherein at least a portion of the first plurality of interrogatingpulses generate a first plurality of backscattered pulses and at least aportion of the second plurality of interrogating pulses generate asecond plurality of backscattered pulses; receiving the first pluralityof backscattered pulses and the second plurality of backscattered pulsesat a detector; collecting the first plurality of backscattered pulsesand the second plurality of backscattered pulses in a storage medium;and analyzing the first plurality of backscattered pulses and the secondplurality of backscattered pulses, the analyzing comprising the stepsof: identifying a pulse profile associated with each of the firstplurality of backscattered pulses and the second plurality ofbackscattered pulses, and identifying intensity values in each of thefirst plurality of backscattered pulses and the second plurality ofbackscattered pulses corresponding to a first plurality of locations inthe detection area, for each of the first plurality of locations,performing the steps of: constructing a first trace of the intensityvalues from the first plurality of backscattered pulses, where eachintensity is associated with the pulse profile of a correspondinginterrogating pulse of the first plurality of interrogating pulses,constructing a second trace of the intensity values from the secondplurality of backscattered pulses, where each intensity is associatedwith the pulse profile of a corresponding interrogating pulse of thesecond plurality of interrogating pulses, comparing the first trace toat least the second trace, quantifying a correlation between the firsttrace and the second trace, and identifying a first location of a firstfast physical disturbance in the detection area causing a differencebetween the first trace and the second trace.
 4. The method of claim 3,wherein the quantifying the correlation includes analyzing the firsttrace and the second trace based on intensity as a function ofwavelength to determine the correlation between the first trace and thesecond trace.
 5. An apparatus for detecting a physical disturbancecomprising: a coherent light source transmitting a light wave withperiodically changing parameters, the light source generating a firstsweep cycle including a first plurality of interrogating pulses and atleast a second sweep cycle including a second plurality of interrogatingpulses; a first optical fiber of a predetermined length disposed in adetection area, wherein the first optical fiber receives the first sweepcycle and the second sweep cycle and reflects at least a portion of thefirst sweep cycle as a first backscattered sweep cycle including a firstplurality of backscattered pulses and at least a portion of the secondsweep cycle as a second backscattered sweep cycle including a secondplurality of backscattered pulses; and a detector configured to: receivethe first backscattered sweep cycle and the second backscattered sweepcycle from the first optical fiber; beginning with the firstbackscattered sweep cycle and the second backscattered sweep cycle,perform iterative steps of: identifying a pulse profile associated witheach of the first plurality of backscattered pulses and the secondplurality of backscattered pulses, identifying intensity values in eachof the first plurality of backscattered pulses and the second pluralityof backscattered pulses corresponding to a first plurality of locationsin the detection area, constructing a first trace of the intensityvalues from the first plurality of backscattered pulses, where eachintensity is associated with the pulse profile of a correspondinginterrogating pulse of the first plurality of interrogating pulses,constructing a second trace of the intensity values from the secondplurality of backscattered pulses, where each intensity is associatedwith the pulse profile of a corresponding interrogating pulse of thesecond plurality of interrogating pulses, comparing the first trace ofthe first backscattered sweep cycle to at least the second trace of thesecond backscattered sweep cycle, quantifying a correlation between thefirst backscattered sweep cycle and the second backscattered sweepcycle, and identifying a first location of a first fast physicaldisturbance in the detection area causing a difference between the firsttrace and the second trace.
 6. The apparatus of claim 5, wherein thequantifying the correlation includes analyzing the first backscatteredsweep cycle and the second backscattered sweep cycle based on intensityas a function of wavelength to determine the correlation between thefirst backscattered sweep cycle and the second backscattered sweepcycle.